Abstract Elimination of excess dendritic spines on cortical pyramidal neurons during adolescence is critical for excitatory/inhibitory (E/I) balance in adult cortical circuits, and its impairment can lead to altered spine density in autism spectrum disorders (ASD) and schizophrenia. This proposal seeks to illuminate a novel mechanism for dendritic spine remodeling in the developing mammalian frontal cortex, mediated by immunoglobulin (Ig)- class cell adhesion molecules of the L1 family and class 3 Semaphorins. Aims focus on NrCAM (Neuron-Glial Related Cell Adhesion Molecule) and Close Homolog of L1 (CHL1), which are associated with ASD, schizophrenia, and intellectual disability. The central hypothesis to be investigated is that NrCAM and CHL1 form holoreceptor complexes with Neuropilin1/2 and PlexinAs for secreted class 3 Semaphorins, which signal through Rho GTPases to prune dendritic spines of cortical pyramidal neurons during adolescence. Aim 1. Developmental Regulation of NrCAM and Role of CHL1 in Dendritic Spine Remodeling A new conditional mutant mouse (Nex1Cre-ERT2: NrCAMf/f) that inducibly deletes NrCAM from cortical pyramidal neurons will be studied to define the developmental timing of NrCAM function in dendritic spine morphogenesis, spine dynamics, and cortical excitability. A novel function for CHL1 in spine remodeling will be identified by analysis of spine/synapse morphogenesis and cortical excitability in CHL1 null mutant mice. Aim 2. Structural and Functional Interactions of the Sema3F Holoreceptor A structure-function approach using mutagenesis will be undertaken to probe an innovative molecular model of the Sema3F holoreceptor complex, in which the NrCAM extracellular domain interacts with Npn2 to induce Sema3F-induced receptor clustering and spine remodeling. NrCAM cytoplasmic domain interactions with Synapse-associated protein 102 (SAP102) and cytoskeletal adaptors (Ankyrin-B, -G, and Doublecortin-like kinase 1) will be analyzed for promoting receptor clustering and signaling at the nascent postsynaptic density in cortical neuronal cultures and in vivo. Aim 3. Molecular Mechanism of Sema3F Signaling through Small GTPases A novel dual signaling pathway will be investigated in which Sema3F-induced signaling through RhoA (Rho Kinase-Myosin II) generates contractile force that exerts tension on actin filaments assembled through Rac1 signaling (Tiam1-Rac1-PAK-LIMK1-Cofilin1) to regulate spine elimination/protrusion. The role of intrinsic activity in Sema3F-mediated spine pruning will be evaluated by activity blockade in cortical neuron cultures. This project is expected to have sustained overall impact as it will delineate a novel molecular mechanism for regulating excitatory synapse development in the frontal neocortex, advance mechanistic understanding into pathology associated with neurodevelopmental disorders, and may reveal new therapeutic targets for intervention in adolescence, a window of opportunity for influencing cortical networks.